1. Field of the Invention
The present invention relates to an exhaust turbine supercharger, and more particularly to a motor-driven supercharger having an electric motor built-in.
2. Description of Related Art
To supply an air having a density increased by a compressor to an engine is called supercharging, and a structure providing for a driving work of the compressor on the basis of an exhaust energy is called as an exhaust-gas turbocharger.
The exhaust-gas turbocharger is generally constituted by a compressor and a turbine which are arranged so as to position a bearing unit therebetween. The compressor has a compressor impeller built-in, and the turbine has a turbine impeller built-in, respectively. The compressor impeller and the turbine impeller are coupled to each other by a connecting shaft supported by a bearing unit, and are structured such as to rotationally drive the turbine impeller by an exhaust gas of the engine, transmit the rotating force to the compressor impeller via the connecting shaft, and compress the air by the compressor impeller so as to supercharge to the engine.
In the exhaust turbine supercharger mentioned above, there has been already proposed a structure having an electric motor built-in for assisting an acceleration at a time of rotating at a low speed (for example, patent documents 1 and 2). Hereinafter, the exhaust-gas turbocharger having the electric motor built-in mentioned above is simply called as “motor-driven supercharger”.
A turbocharger in the patent document 1 is provided with a power generator constituted by a power generating and motor driving rotor 54 attached onto a shaft 53 coupling a turbine 51 and a compressor 52, and a stator 56 attached within a housing 55, and a cooling water jacket 57 formed in an inner portion of a housing surrounding the stator for cooling the stator, as shown in FIG. 1.
The patent document 2 relates to a motor-driven compressor. As shown in FIG. 2, the motor-driven compressor is provided with a compressor housing 61, an electric motor 62 and a centrifugal compressor wheel 63. The compressor housing 61 forms a compressed air outlet positioned in an outer periphery, and an outer peripheral casing 61a and an inner motor support 61b of the compressor housing form a flow path introducing an ambient air from a portion therebetween.
Patent Document 1:
Japanese Unexamined Patent Publication No. 2000-130176 “TURBOCHARGER PROVIDED WITH GENERATOR AND MOTOR”
Patent Document 2:
U.S. Pat. No. 5,904,471 “COOLING MEANS FOR A MOTOR-DRIVEN CENTRIFUGAL AIR COMPRESSOR”
A rotating speed of the exhaust-gas turbocharger is high, and comes up to, for example, at least 100 to 200 thousand rpm. Accordingly, the electric motor installed in the motor-driven supercharger mentioned above is demanded such that a rotational drive and a regenerative operation can be executed at a high speed in correspondence to the high-speed rotation.
In order to satisfy the demand, in the case that a brushless ac motor is employed as the electric motor, it is absolutely necessary to employ an inverter converting a dc power of a battery mounted to a vehicle into an ac power having an optional frequency.
However, since the electric power after converted by the inverter is the ac power, there is a problem that a great power loss is generated due to an electric resistance and an inductance of a power supply cable connecting the inverter and the motor-driven supercharger. Accordingly, particularly in the case that the frequency is high, it is necessary to supply a greater electric power than an electric power inherently consumed by the electric motor from the inverter, so that there is a problem that the inverter itself is enlarged in size in addition to the great loss.